WClark

The wifes S8 with the nes BBS CH just got trucked 60 miles to the dealer.

She had driven about 40 miles north of here when she said the temp gauge went way up. She parked it and it seems to be out of coolant. I checked it Friday and it was fine. I got there about the time the AAA truck was pulling it onto the flatbed. No signs of coolant leaking. Must be a head gasket.

I must say the Audi Roadside assistance was good and the Northern New England AAA that responded was excellent. I guess Audi usually only authorizes it to be taken it to the nearest dealer but in Southern New Hampshire, none of them are nearby so Audi agreed to bring it to Reitzl in Norwell Mass, the dealer we bought it from which was an additional 40 miles or so.

PaulW

If it didn't have coolant in the car, the low level coolant alarm would of gone off. If it was a head gasket, there would of been steam coming out of the tailpipe with a noticable loss of power (I think).

If the coolant temperature went way up, it might of been from the fan not working....mine does that when I don't turn my ignition switch back, which picks up the relay to energize the fan.

You changed the wheels correct? So I doubt anything you did caused this, but you never know....when something doesn't work on my car, I usually go back and figure out what was the last thing I worked on....I've caused a lot of simple problems....it's a good troubleshooting technique.

WClark

...The dealer called. It seems the ATF cooler in the engine radiator let go and the tranny sucked up a lot of - but not all the engine coolant. The engine is probably fine, just need to be flushed. They have to install a new transmission along with a new radiator.

He said if this was an older car (not on warranty) they would recommend flushing the tranny and seeing how it works out, but not with a new one.

PaulW

I don't know if I could troubleshoot that in four hours, but maybe? If I did, I sure would check my diagnostics about 20 times. Never heard of a problem like this.

I would almost think that the ATF pressure in the lines would far exceed the pressure of the engine cooling system, therefore, the ATF would leak into the engine system, not vice versa.

What pressure does the engine system run at? About 220F, closed, pressurized system, that can't be more than 15-30 PSI, if so, the radiator hoses would burst.

All of the tranny lines are tubing, correct? If so, I bet they are rated for more like 100 PSI. I personally don't know about the Audi's, I'm just generalizing, but does this make sense or not?

Randy and Skiracer, what do you think? I could be thinking wrong here.

pw

ryoung

.
If there's just a small crack in tubing that has high liquid flow rates, the high pressure fluid aspirates the lower pressure fluid into the line.

Saw it once with my own eyes in a system with a 50 psi differential the "wrong" way. Strange, but true.

skiracer

My guess would be that the cooler would be on the high-pressure side of the pump, but I suppose that it could be on the suction side. Depending on that, the pressure in the line could be relatively high (higher than 100 psi- most hydraulic systems run in the thousands of psi but these trannys likely run at a 300-500 psi- not sure though), or really low if on the suction side of the pump (5-20 psi). ryoung is correct about high-flow lines being able to asperate liquids from a low pressure- called the venturi effect, Bournoulli's equation describes this (it's a conservation of energy thing). But the velocity required to overcome a possible 450 psi delta would be huge! Personally, I would prefer having to lose the tranny fluid not the engine coolant- although either one would suck (pun intended), the tranny is likely cheaper to fix/replace than the engine. My sympathies go out to WClark- fortunately his car is still under warranty. I've never heard of this one before, and I hope to never hear it again!

PaulW

If so, the radiator would be at something like 24" wg (inches of water) which is what? 2 lbs? Believe it or not, I may not be able to spell Bernulli's name, but I did study it and yes, I agree that a venturi effect could possibly do that. However, to a system that's at 2 PSI? Sounds hard to believe. I agree that the cooler would be on the high pressure side of the pump (discharge) not the suction side.

WClark, did they drain water/coolant out of the tranny? When they flushed the engine, did it have oil in it?

Can I buy that "now defective tranny" from you/your Audi dealer, why not just send it to me, I'll pay the shipping :-).

I could get a 2000 or newer tranny ECU, a tip shift plate for the car and retrofit it....

skiracer

after it goes through the engine. However, the entire cooling system becomes pressurized as the engine heats up in order to allow higher operating temps without boiling the water. So the 24" of pressure only exisits when the motor is cold. But still, trying to coax 15-30 psig water into a 300-500 psig line would be a major trick. I need to look at the system to be able to figure this one out. By the way, if you get that "defective" tranny, you will need to change out the wiring harness, too (rumour has it, anyway).

skiracer

The water in the engine coolant system WOULD be at a higher pressure right after engine shutoff! When the transmission fluid pump stops running, the pressure drops to near zero. However, the coolant pressure remains high for quite a while, until it cools down (which is a very long time for these 4.2L engines!) Thus, the delta pressure between the water and the transmission fliud would be a positive 15-30 psig! The question is, why didn't the tranny fluid run into the water first? Perhaps the tranny cooler core failed just before engine shutdown. WClark- how long had your wife been driving when the overtemp light illuminated?

PaulW

That is exactly what could be going on. Nice job Einstein!